Soybean proteins having various protein contents are produced from defatted soybeans. For example, powdered defatted soymilk having a relatively low protein content is obtained by removing insoluble materials, i.e., so-called "okara" from defatted soybeans. Concentrated soybean protein the protein content of which is higher than powdered defatted soymilk is obtained by removing whey component from defatted soybeans. Isolated soybean protein which has a most highest protein content is obtained by removing both "okara" and whey component from defatted soybeans. These soybean proteins are used for various purposes.
Thus, defatted soybeans are very important raw materials in the production of various soybean proteins.
In order to obtain soybean protein having good taste, defatted soybeans having good taste are required. For this purpose, various developments and studies have been done and it has been found that lipoxigenase participates in formation of so-called grassy-flavor which adversely affects taste of defatted soybeans. Then, there have been developed many processes for producing soybean protein by inactivating or inhibiting lipoxigenase.
In practice, inactivation of lipoxigenase is generally effected by heating. In this case, as lipoxigenase is inactivated, soybean protein is also denaturated with heating and, thereby, NSI of the resulting defatted soybeans are lowered. Therefore, when the protein is extracted with an aqueous solvent such as in the case of the production of isolated soybean protein or soybean curd (so-called "tofu"), there is a problem that yield is lowered. That is, although soybean protein having good taste can be obtained by inactivation of lipoxigenase with heating, yield of soybean protein is lowered. Further, such a heating requires very complicated temperature control.
If it is necessary to make great account of yield, defatted soybeans having high NSI should be used. However, in defatted soybeans having high NSI, heat denaturation of protein is little and, naturally, lipoxigenase is not inactivated, which results in bad taste of defatted soybeans, or soybean protein and "tofu" obtained therefrom.
As means for increasing in yield of soybean protein, various extraction processes such as alkali extraction of defatted soybeans and extraction using a reducing agent have been known. However, there is a problem of bad taste due to formation of alkaline odor and the like.
Thus, hitherto, it has been considered that, when NSI of defatted soybeans is lower, taste thereof is better. In other words, defatted soybeans having both high NSI and good taste have been hardly obtained and it has been difficult to obtain soybean protein having good taste in a high yield.
Further, in the case of concentrated soybean protein wherein an aqueous acidic solvent is used, there is another problem that, when NSI is low, emulsifiability and gelation properties become inferior. Furthermore, when NSI is low, there is still another problem that, in the case of the production of textured soybean protein, soybean protein is hardly texturized.
On the other hand, recently, it has become feasible to produce a new variety of soybeans which lacks some of its lipoxigenase isozymes, simultaneously, and it has been proposed to use the variety in the production of soybean products having less grassy-flavor.
For example, the Japanese newspaper, Nikkei Sangyo Shinbun dated Nov. 8, 1986, discloses an article relating to commercialization (processing and production of soymilk yoghurt, "tofu", fermented soybeans (so-called "natto"), etc.) of a new variety, "pleasant soybeans", free from grassy-flavor and bitterness. In Japanese Patent Kokai No. 61-170359, there is disclosed raw materials for food (soymilk) and food products (processed soymilk products) made from soybeans lacking lipoxigenase isozymes L-2 and L-3. Further, the Kokai No. 61-170359 also discloses bean sprouts of the soybeans lacking lipoxigenase isozymes L-2 and L-3 in which grassy-flavor is reduced. K. Kitamura, Nippon Nogei Kagaku Kaishi, "Removal of unpleasant flavor of soybeans by genetic breeding", 59, (10) 1071-1078 (1985) discloses that several series of soybeans lacking lipoxigenase isozymes L-2 and L-3 are obtained and almost grassy-flavor can be removed. Further, K. Kitamura, Nippon Shokuhin Kogyo Gakkaishi 31, (11), 751-758, (1984) discloses breeding of lipoxigenase lowering soybeans. Japanese Patent Kokai No. 61-56027 and U.S. patent application Ser. No. 769,235 filed Aug. 26, 1985 also disclose a process for producing new variety of odorless soybeans which lack lipoxigenase isozymes.